I had such a hard time with my Laptop sound for last 5 months, I almost gave up. This is a Toshiba Tecra M5 notebook on Windows XP SP2. It has SigmaTel HD Audio onboard sound card. Opening up service tickets with the producer of my lifeline did not help. There were several problems in different avatars. In the beginning it was this Auto Installed 'Microsoft Kernal Audio *' devices which were creating the problems. It could be any one of MS Kernel Audio devices - Mixer, Echo Canceler or Synthesizer.   From  the middle of nowhere it would  appear and sit under my Device Manager for Sounds with an uncertain Yellow Tag. It means that the MS device drivers were not auto installed properly.  When the sound disappears, I would remove the drivers (good, bad and ugly ones!)  for sound  under 'Device Managers' and reinstall just the driver (latest from Toshiba support site) for SigmaTel. Everything would be hunky dory for a few days. Again, like a Virus one of these MS Kernel Audio * would appear from nowhere and sit under the Device Manager for Sound,  with an Yellow Tag. After a tinkering for few more months, alas!, all my sound disappeared (except for the system sounds). I almost gave up at this point. No amount of Tinkering helped.
Then one day, one of the internet forums throwed some light. It suggested a way to show the hidden non present device drivers. To get to it,  add a System Variable (not User Variable)  'devmgr_show_nonpresent_devices' and set its  value to '1'.  You can get to System Variables from 'System Properties' and 'Advanced' Tab. Make sure to select 'View > Show hidden devices' from Device Manager. That showed some of the hidden Sound devices under Device Manager.  But how to spook them out. It would not go away after uninstalling the devices. I then remembered one of old tricks I did for Windows 98 and 2000. Run Windows in Safe Mode and remove the Device Drivers.  In fact, I removed all of the Sound Devices while running Windows XP in safe mode. This included all of the Microsoft Kernel Audio devices as well. I then reinstalled the SigmaTel HD Audio driver and restarted Win XP. Wow, I can listen to all my MP3s now !. All of  Microsoft Kernel Audio * device drivers got installed automatically without any problems. I saw a lot of emails out there with the same sound issue, hence I thought to blog about it.